1. Field of the Invention
The present invention relates to a power module device and, more particularly, to a power module device which combines a power element such as a MOSFET, and a control element therefor into one piece.
2. Description of the Related Art
FIG. 11 is a top view of a known power module device with the exterior cover removed. FIG. 12 and FIG. 13 are a side view and a perspective view, respectively, of the power module device of FIG. 11 which has been mounted so as to be used practically. As shown in FIG. 11, the whole surface of a metallic substrate 1 is covered with an insulating resin layer (not shown) made of epoxy resin or the like, and wiring patterns 5 are provided on the insulating resin layer. Provided on the wiring patterns 5 are a heat sink 2 on which a MOSFET 3 constituting a power element is mounted. The MOSFET 3 is electrically connected to the wiring pattern 5 on the metallic substrate 1 through a bonding wire 6 or the like made of a fine metallic wire as illustrated. Mounted on the metallic substrate 1 is a control element 4 for controlling the MOSFET 3, the control element 4 being electrically connected to the MOSFET 3 through the wiring pattern 5 and the bonding wire 6. The MOSFET 3, the heat sink 2, etc. are covered with gel-like resin (not illustrated) to protect them against humidity. A relatively viscous gel-like resin is suitable for this purpose so that it does not spread too much because it is only locally used for the MOSFET 3 on the metallic substrate 1. Further, since lead wires 9, which are electrically connected to the wiring patterns 5 on the metallic substrate 1, are connected to electrodes 106 (see FIG. 12), etc. provided on a printed circuit board 100 (see FIG. 12) at the time of mounting; therefore, the lead wires 9 are provided so that it extends outward in a cantilever-manner. The metallic substrate 1 is mounted on a radiator 102 (see FIG. 12) such as a heat sink and therefore the metallic substrate 1 is provided with a plurality of mounting holes 7 for installing it on the radiator 102.
In the case of the known power module device having the structure described above, as illustrated in FIG. 12 and FIG. 13, a box-shaped cover 108 made of resin is provided on the metallic substrate 1 for mechanically protecting the MOSFET 3 and other mounted components including the control element 4 at the time of packaging. The box-shaped cover 108 is installed in such a manner that only both ends of the metallic substrate 1, where the mounting holes 7 of the metallic substrate 1 are provided, are exposed. The metallic substrate 1 is attached to the radiator 102, such as a heat sink, by screws 70 installed through the mounting holes 7. The radiator 102 is attached on the printed circuit board 100, such as a glass-epoxy substrate, by screws 22 as illustrated. Furthermore, a partitioning plate 108b is provided within the box-shaped cover 108 as illustrated in FIG. 12 to mechanically protect the lead wires 9 which extend in a cantilever-manner. The wirings 5 extend under the partitioning plate 108b (see FIG. 11) and ends thereof are soldered to the lead wires 9. The lead wires 9 extend in parallel to the metallic substrate 1, pass through the printed circuit board 100, and are electrically connected, with solder 16, to for example the electrodes 106 mounted on the printed circuit board 100. For the purpose of reinforcing the lead wires 9, the space which is enclosed by the box-shaped cover 108, the partitioning plate 108b, and the metallic substrate 1, is filled with resin 104.
As stated above, the known power module device has the MOSFET 3 and the control element 4 which are mounted two-dimensionally on the single metallic substrate 1. Further, the conductor constituting the wiring pattern 5 needs to be relatively thick because a relatively large current must be supplied to the wiring pattern 5, this results because it is difficult to provide the wiring pattern 5 with a high-density wiring. For these reasons, the known metallic substrate 1 has a problem in that it needs to be made large for the wiring and therefore it is extremely difficult to make the device smaller.
In addition, there are problems related to the lead wires. The lead wires 9, which extend outward in a cantilever manner, require the box-shaped cover 108 and the resin 104 or the like filled in the box-shaped cover 108 to provide the lead wires 9 with a mechanical protection. Further, protection of the components including the MOSFET 3 and the wiring pattern 5 mounted on the metallic substrate 1 from mechanical damage requires the provision of the box-shaped cover 108 made of resin or the like to cover the surface of the metallic substrate 1. Therefore, numerous assembly steps are required, resulting in a high cost production.
There is still another problem with the conventional power module device namely, the heat generated by the MOSFET 3 constituting the power element is transmitted to the whole metallic substrate 1 through the heat sink 2, and the control element 4 is unavoidably subjected to the heat, causing the control element 4 to malfunction.